Ultrasonic sensor having edge-based echo detection

ABSTRACT

Sensors may employ a constant false alarm rate (CFAR) screening process in combination with edge-based echo detection. In one illustrative embodiment, a sensor controller includes: a transmitter, a receiver, and a processing circuit coupled to the transmitter and to the receiver. The transmitter drives a piezoelectric element to generate acoustic bursts. The receiver senses a response of the piezoelectric element to echoes of each acoustic burst. The processing circuit is operable to apply echo-detection processing to the response by: determining a derivative signal from the response; and detecting an echo based at least in part on a peak in the derivative signal indicating a rising and/or falling edge in the response. Signaling to the electronic control unit may specify a time of flight associated with each edge.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/808,947, filed on Feb. 22 2019, the entire contentsof which is incorporated herein by reference.

BACKGROUND

Modern automobiles are equipped with an impressive number and variety ofsensors. For example, cars are now routinely equipped with arrays ofultrasonic sensors to monitor distances between the car and any nearbypersons, pets, vehicles, or obstacles. Due to environmental “noise” andsafety concerns, each of the sensors may be asked to provide tens ofmeasurements each second while the car is in motion. It is important forsuch sensor arrays to perform reliably, even in environments that changein complex ways. Seemingly small differences, such as the presence orabsence of a curb, or even the difference between paved and gravelsurfaces, can significantly change the characteristic reflection of apole, bollard, or other slim obstacle. Previous attempts to identify theenvironment and responsively tailor the detection process have proven tobe computationally prohibitive and/or inadequate.

SUMMARY

Accordingly, there are disclosed herein various sensors, sensorcontrollers, and sensor control methods employing a constant false alarmrate (CFAR) screening process in combination with edge-based echodetection. In one illustrative embodiment, a sensor controller includes:a transmitter, a receiver, and a processing circuit coupled to thetransmitter and to the receiver. The transmitter drives a piezoelectricelement to generate acoustic bursts. The receiver senses a response ofthe piezoelectric element to echoes of each acoustic burst. Theprocessing circuit is operable to apply echo-detection processing to theresponse by: determining a derivative signal from the response; anddetecting an echo based at least in part on a peak in the derivativesignal indicating a falling edge in the response.

In another illustrative controller embodiment, the processing circuit isoperable to apply echo-detection processing to the response by:determining a derivative signal from the response; and detecting echoesbased at least in part on peaks in the derivative signal indicatingedges in the response.

In an illustrative method embodiment, a piezo-electric based sensor isoperated by: driving a piezoelectric transducer to generate a burst ofacoustic energy during an actuation interval; during a measurementinterval following the actuation interval, obtaining a response of thepiezoelectric transducer; and processing the response to sense echoes ofthe burst. The processing includes: determining a derivative signal fromthe response; and detecting echoes based at least in part on peaks inthe derivative signal indicating edges in the response.

Each of the foregoing embodiments may be employed together with any oneor more of the following optional features: 1. detecting an echo isfurther based on: comparing the response to an adaptive threshold toproduce a comparator signal; and detecting an edge in the comparatorsignal. 2. the processing further includes deriving the adaptivethreshold from the response signal using a constant false alarm rate(CAFR) process. 3. the derivative signal is determined by: forming adifference signal between the response signal and a delayed responsesignal; taking an absolute value of the difference signal; and applyinga local filter to the absolute value of the difference signal. 4. thederivative signal is further determined by: applying a background filterto the absolute value of the difference signal; and taking a ratio of anoutput from the local filter to an output of the background filter. 5.the processing further includes: generating a pulse on an I/O line tosignal a time of flight of the echo. 6. the processing further includes:generating a data frame that includes, for each echo detected inresponse to an acoustic burst, a time of flight for a falling edge ofthat echo. 7. the processing further includes signaling an electroniccontrol unit to indicate one or more times of flight associated witheach echo, the one or more times of flight each corresponding to one ofsaid peaks in the derivative. 8. the processing further includessignaling two times of flight associated with each echo, with a firsttime of flight corresponding to a rising edge peak and a second time offlight corresponding to a falling edge peak. 9. measuring peak amplitudeof each echo. 10. adjusting the one or more times of flight associatedwith each echo based at least in part on the peak amplitude of thatecho. 11. adjusting is based on a ratio between the peak amplitude ofthat echo and a peak in the derivative for an edge of that echo.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overhead view of an illustrative vehicle equipped withparking-assist sensors.

FIG. 2 is a block diagram of an illustrative parking assist system.

FIG. 3 is a circuit schematic of an illustrative parking-assist sensor.

FIG. 4A is a graph relating controller input and output signals fordifferent existing interface standards.

FIG. 4B is a graph of illustrative signals for an enhanced edge-basedecho detection process.

FIG. 5 is an illustrative CFAR implementation.

FIGS. 6A and 6B are block diagrams of an illustrative edge-based echodetection process.

FIG. 7 is a graph for an illustrative time correction look up table.

DETAILED DESCRIPTION

It should be understood that the drawings and following description donot limit the disclosure, but on the contrary, they provide thefoundation for one of ordinary skill in the art to understand allmodifications, equivalents, and alternatives falling within the scope ofthe claim language.

As an illustrative usage context, FIG. 1 shows a vehicle 102 equippedwith a set of ultrasonic parking-assist sensors 104. The number andconfiguration of sensors in the sensor arrangement varies, and it wouldnot be unusual to have six sensors on each bumper with two additionalsensors on each side for blind-spot detectors. The vehicle may employthe sensor arrangement for detecting and measuring distances to objectsin the various detection zones, potentially using the sensors forindividual measurements as well as cooperative (e.g., triangulation,multi-receiver) measurements.

The ultrasonic sensors are transceivers, meaning that each sensor cantransmit and receive bursts of ultrasonic sound. Emitted burstspropagate outward from the vehicle until they encounter and reflect froman object or some other form of acoustic impedance mismatch. Thereflected bursts return to the vehicle as “echoes” of the emittedbursts. The times between the emitted bursts and received echoes areindicative of the distances to the reflection points. In many systems,only one sensor transmits at a time, though all of the sensors may beconfigured to measure the resulting echoes. However multiplesimultaneous transmissions can be supported through the use oforthogonal waveforms or transmissions to non-overlapping detectionzones.

FIG. 2 shows an electronic control unit (ECU) 202 coupled to the variousultrasonic sensors 204 as the center of a star topology. Of course,other topologies including serial, parallel, and hierarchical (tree)topologies, are also suitable and contemplated for use in accordancewith the principles disclosed herein. To provide automated parkingassistance, the ECU 202 may further connect to a set of actuators suchas a turn-signal actuator 206, a steering actuator 208, a brakingactuator 210, and throttle actuator 212. ECU 202 may further couple to auser-interactive interface 214 to accept user input and provide adisplay of the various measurements and system status. Using theinterface, sensors, and actuators, ECU 202 may provide automatedparking, assisted parking, lane-change assistance, obstacle andblind-spot detection, and other desirable features.

One potential sensor configuration is now described with reference toFIG. 3. (Other communication and power supply techniques such as thoseprovided in the DSI3, LIN, and CAN standards, would also be suitable andare contemplated for use in accordance with the principles disclosedherein.) Besides the two power terminals (Vbat and GND) shown in theembodiment of FIG. 3, each of the illustrative ultrasonic sensors isonly connected to the ECU 202 by a single input/output (“I/O” or “I/O”)line. The I/O line may be biased to the supply voltage (the“de-asserted” state) by a pull-up resistor when it is not activelydriven low (the “asserted” state) by the ECU 202 or by the sensorcontroller 302. The communication protocol is designed to have only oneof the two controllers (ECU 202 or sensor controller 302) asserting theI/O line at any given time.

The sensor controller 302 includes an I/O interface 303 that, whenplaced in a recessive mode, monitors the I/O line for assertion by theECU 202 and, when placed in a dominant mode, drives the state of the I/Oline. The ECU communicates a command to the sensor by asserting the I/Oline, the different commands being represented by assertions ofdifferent lengths. The commands may include a “send and receive”command, a “receive only” command, and a “data mode” command.

The sensor controller 302 includes a core logic 304 that operates inaccordance with firmware and parameters stored in nonvolatile memory 305to parse commands from the ECU and carry out the appropriate operations,including the transmission and reception of ultrasonic bursts. Totransmit an ultrasonic burst, the core logic 304 is coupled to atransmitter 306 which, with a suitably modulated local oscillator signalfrom a voltage controlled oscillator 307, drives a set of transmitterminals on the sensor controller 302. The transmitter terminals arecoupled via a transformer M1 to a piezoelectric element PZ. Thetransformer M1 steps up the voltage from the sensor controller (e.g., 12volts) to a suitable level for driving the piezoelectric element (e.g.,tens of volts). The piezoelectric element PZ has a resonance frequencythat is tuned to a desirable value (e.g., 48 kHz) with a parallelcapacitor C3, and has a resonance quality factor (Q) that is tuned witha parallel resistor R1. One illustrative purpose of the tuning capacitorand tuning resistor is to tune the parallel resonance frequency close tothe series resonant frequency of the piezoelectric element.

As used herein, the term “piezoelectric transducer” includes not onlythe piezoelectric element, but also the supporting circuit elements fortuning, driving, and sensing, the piezoelectric element. In theillustrative embodiment, these supporting elements are the transformerM1, the tuning resistor and tuning capacitor, and the DC-isolationcapacitors. Optionally, output and input capacitance of the transmitter306 and amplifier 308, respectively, may also be included as parasiticcharacteristics of the supporting circuit elements considered to be partof the transducer. However, the use of the term “piezoelectrictransducer” does not necessarily require the presence of any supportingcircuit elements, as a piezoelectric element may be employed alonewithout such supporting elements.

A pair of DC-isolation capacitors C1, C2 couple the piezoelectricelement to the sensor controller's pair of receive terminals to protectagainst high voltages. Further protection is provided with internalvoltage clamps on the receive terminals. Such protection may be desiredfor the intervals when the piezoelectric element is transmitting. As thereceived echo signals are typically in the millivolt or microvolt range,a low-noise amplifier 308 (also referred to herein as a “front-endamplifier”) amplifies the signal from the receive terminals. An optionalmixer 309 multiplies the amplified receive signal with the localoscillator signal to downconvert the modulated signal to baseband beforeit is digitized and processed by a digital signal processor (DSP) 310with an integrated analog-to-digital converter (ADC).

DSP 310 applies programmable methods to monitor the piezoelectrictransducer during the transmission of a burst, and to detect any echoesand measure their parameters such as time-of-flight, duration, and peakamplitude. Such methods may employ threshold comparisons, minimumintervals, peak detections, zero-crossing detection and counting, noiselevel determinations, and other customizable techniques tailored forimproving reliability and accuracy. The DSP 310 may further process theamplified receive signal to analyze characteristics of the transducer,such as resonance frequency and quality factor, and may further detecttransducer fault states.

Commands received via the I/O line trigger the core logic 304 to operatethe transmitter and receiver and to provide the measurement results tothe ECU 202 via the I/O line, as explained further below. In addition tothe echo measurements and transducer fault states that may be detectedby the DSP 310, the core logic may monitor other sensor conditions suchas having the supply voltage “under-voltage” or “over-voltage” whiletransmitting an ultrasonic burst, thermal shutdown of transmitter, ahardware error, an incomplete power-on reset, or the like. The corelogic 304 may detect and classify multiple such transducer fault statesand error conditions, storing the appropriate fault codes in internalregisters or nonvolatile memory 305.

FIG. 4A provides some illustrative signal timing to aid in understandingthe operation of the illustrative sensor embodiments, particularly withregard to communication on the I/O line. An ECU formulates a signalpulse “CMD” having a duration that represents a desired command. In thisinstance the duration is “Ts” to represent a “send and receive” command.(Illustrative command pulse durations may be in the 300-1300 microsecondrange.)

Signal graphs 101, 102, and 103, show potential uses of the I/O line inaccordance with different existing standards, which may be respectivelytermed “standard”, “pulse echo reporting”, and “advanced”. During a time400 when the sensor is inactive (i.e., not performing a measurement orotherwise responding to a command from the ECU), the I/O line is high(de-asserted) by default. During this time 400, the ECU is allowed tocontrol the I/O line. As can be seen with each of the signal graphs 101,102, and 103, the ECU asserts the IO signal by actively driving the linelow for the duration representing the command. There is a smallpropagation delay due to limited slew rates on the I/O line, and adebounce interval (“T_(DB)”) follows the assertion and de-assertion toensure that the timing of line's return to battery voltage is deliberateand not a result of transient noise. (Illustrative debounce intervalsmay be in the 40-80 microsecond range.)

With the lapse of the debounce interval, the sensor controller decodesthe command and takes control of the I/O line for a predeterminedinterval 401 that may depend on the command. For a “send and receive”command, the sensor controller begins the predetermined interval 401with the transmission of an acoustic burst 402 and retains control untila programmed measurement interval has elapsed. Before discussing theoperation of the I/O line during this measurement interval 401, weconsider the operation of the piezoelectric transducer and thecorresponding amplified receive signal RX.

The operation of the piezoelectric transducer is here represented as avibration signal VIBR representing mechanical oscillation of thepiezoelectric element. (Note that the signal is not shown to scale, asthe transmitted burst 402 may be orders of magnitude larger than theechoes 408, 409.) Electrically, the mechanical vibration of thepiezoelectric element can be detected as a voltage or a current. Themechanical vibration amplitude increases as the controller 302 drivesthe transducer (the “driving stage” 404), then decreases after thedriving operation is concluded (the “reverberation stage” 406). Thecontroller 302 may employ active and/or passive damping to reduce theduration of the reverberation stage.

In the sensor embodiment of FIG. 3, the vibration is detected as aclamped, amplified version of the secondary voltage VX via amplifier308. For explanatory purposes the RX signal illustrated in FIG. 4 is a(low-pass filtered) envelope of this clamped, amplified voltage signal,but the amplified oscillatory signal can also be employed.

The sensor controller measures a noise level during a pre-transmitperiod 403, which may begin one debounce interval after the ECU assertsthe I/O line and may end when the transmit burst is sent. The actuationof the transducer for the transmit burst causes the RX signal tosaturate. (In at least some implementations, internal voltage clamps onthe receive terminals of the sensor controller prevent excessivevoltages from reaching amplifier 308). The transmit burst overwhelms thereceiver and prevents any meaningful echo measurements from beingacquired during this interval. While the receive signal is above athreshold 411 (and/or compliant with other implementation-specificrequirements that aren't relevant here), the standard sensor controllerdrives the I/O line low (101). Thus, the controller asserts the IOsignal during the actuation interval T_(TX), which corresponds to theinterval 412 where the RX signal exceeds the threshold 411. Thisassertion during the transmit burst enables the ECU to measure theactuation interval (“T_(TX)”) of the transducer, enabling it to verifyoperation of the transducer.

Note that the actuation interval 412 includes not only the drive stage404 of the acoustic burst generation, but also a portion of thereverberation stage 406 of the acoustic burst. The time required for thereverberation amplitude to drop below threshold 411 is indicative of thelosses in the transducer, and accordingly may be used as an indicator ofthe quality factor (Q).

Once the receive signal falls below a threshold 411, it becomes possibleto detect echoes, and the standard I/O line (101) is de-asserted untilsuch time as the sensor controller detects a valid echo 418, 419. Therequirements for a valid echo may include, e.g., a minimum time(“T_(DLY)”) above a threshold 411, the minimum time being equal to orgreater than the debounce interval T_(DB). Such a requirementnecessarily requires that the assertion of the I/O line in response toan echo be delayed by the minimum time T_(DLY). The assertion lasts fora duration (“T_(DET)”) equal to the detected length of the echo burst(duration above the threshold 411). In at least some embodiments,multiple echoes may be detected and represented by respective assertionsof the I/O line. At the end of the programmed measurement interval 401,the standard sensor controller 302 releases control of the I/O line.

Sensors employing a pulse reporting protocol (102) employ fixed pulselengths (usually T_(DB)) to report events including a report pulse 422to indicate initiation of the transmit burst 402, a report pulse 423 toindicate the end of the actuation period, and a report pulse 428, 429for each detected echo 418, 419.

Sensors employing an advanced I/O line protocol (103) employ a fixedlength pulse 422 to indicate the beginning of the measurement window401, and a digital data frame 430 to report burst and echo parameters.The burst parameters may include pre-transmit noise level and the lengthof the actuation period. The echo parameter may include time of flight,width, and peak amplitude for each detected echo.

A common factor used by the sensor controllers for each of the protocolsis the echo detection method, which relies on a predetermined threshold411. However, echo envelopes fluctuate in complex ways based on obstacleshape and surroundings, and the fixed-threshold approach is sensitive tosuch variation, limiting the accuracy of obstacle positiondeterminations. Accordingly, there is disclosed herein an enhancededge-based echo detection process.

FIG. 4B again shows the receive signal RX as compared with a dynamicthreshold signal 441 derived using a constant false alarm rate (CAFR)process. A number of CAFR variations exist offering different tradeoffsbetween performance and computational complexity, but in each case theprocesses are intended to keep the probability of detecting a false echoat a relatively constant level even in the presence of varyingbackground noise. The CAFR threshold 441 increases in the presence ofstrong signals and/or noise, and decreases when only weak signals ornoise are present.

When a comparator compares the RX signal to the CAFR threshold 441 inthis example, it produces a pulse 452 to indicate the transmit burst andtwo later pulses 458, 459 to indicate the presence of potential echoes418 and 419. To confirm that the pulses indicate echoes and not noise,the contemplated detection process further estimates a normalizedderivative (DERIV) of the RX envelope signal. In at least somecontemplated embodiments, the RX envelope is a digital signal. Thesensor controller delays the RX signal by one sample interval andsubtracts it from the undelayed RX signal to obtain an estimatedderivative. The estimated derivative may be normalized by taking theabsolute value of each difference and running it through two filters. Afirst, local moving average filter determines a sum or average ofabsolute differences within a small window, the size of which isoptimized for accuracy vs. noise. A second, background filter determinesa weighted sum of absolute differences over a window that is at leastseveral times larger than that of the local filter. The second filtermay be a recursive filter that provides exponential weighting of thepast absolute differences. Each of the filters is preferablyprogrammable to match the bandwidth of filters used for detectingpotential echoes. The normalized derivative (DERIV) is the ratio of thefirst (local) filter output to the second (background) filter output,and it is compared to a predetermined threshold 461. The DERIV curveincludes a peak 462 indicating the rising edge of burst 412, a peak 463indicating the falling edge of burst 412, peaks 464 and 465 indicatingrising and falling edges of echo 418, and peaks 466 and 467 representingrising and falling edges of echo 419. In some alternative embodiments,the background filter is omitted and the filtered derivative signal(rather than the ratio) is compared to the predetermined threshold.Other alternative embodiments use a variable, time-dependent thresholdfor comparison with the filtered or normalized derivatives.

When the sensor controller identifies the presence of a rising orfalling edge of an echo pulse in combination with the receive signalexceeding the CAFR threshold, it reports the presence of an echo to theECU. In at least some contemplated systems, the falling edges tend to bemore abrupt and hence may serve as better indicators of echo presenceand position. Accordingly, the illustrated I/O line signal (104) employsthe pulse reporting protocol with fixed-length pulses positionedbeginning at the falling-edge peaks of the normalized derivative. Pulse472, as with pulse 422, indicates the beginning of a transmit burst.Pulse 473 indicates the falling edge of the transmit burst 412. Pulses478 and 479 indicate the falling edges of the echoes 418, 419. The timeof flight calculation for echoes 418, 419, may take the position ofpulse 473 as the “zero” point.

In alternative embodiments, the fixed pulses are positioned beginning atthe peak of the derivative for the rising edge of the echo pulses. Wherethe rising edge is reported, the time of flight calculation may take thebeginning of the transmit burst or measurement window 401 as the “zero”point.

In other alternative embodiments, the “standard” I/O line protocol isemployed with pulses beginning at the rising-edge peak of the derivativeand ending at the falling-edge peak of the derivative.

Alternatively, the “advanced” I/O line protocol may be employed toreport the time of flight, width, and peak amplitude of each echodetected in accordance with the foregoing principles. Reporting of thedetected echoes can alternatively be performed using other bus protocolssuch as CAN, LIN, DSI3, and PS15.

In each of these embodiments, there may be requirements for the receivesignal to exceed the threshold for a minimum amount of time or for thederivative to exceed the threshold for a minimum amount of time, and ifso, the position of the transitions in the I/O line signal may bedelayed accordingly. In each of these embodiments, the sensor controllermay also or alternatively adjust the time of flight to compensate forany dependence on the echo amplitude as discussed further below.

In one preferred variation of the “advanced” I/O line protocol, the dataframe 430 includes, for each detected echo: (1) a time of flightindicating the position of the rising edge derivative peak; (2) a timeof flight indicating the position of the falling edge derivative peak;and (3) an amplitude of the echo peak.

FIG. 5 is an illustrative implementation of a CAFR process from U.S.Pat. No. 5,793,326 (“Hofele”) which may be adapted for use by the sensorcontroller. A shift register having i blocks, each block having Lsamples of receive signal envelope RX, shifts to accept new sampleblocks from input E. The block in the center is designated the cellunder test (“ZUT”), while the blocks to the left of center formsub-register S1 and the blocks to the right form sub-register S2. Asumming circuit determines for each block the sum of samples within thatblock. A set of maximum value detectors compares the sums pairwise,working outwards from the cell under test, each detector forwarding themaximum sum. A minimum value detector compares the maxima to determinethe smallest one. A divider scales the smallest maximum by L or someother fixed value for normalization, before a multiplier K weights thenormalized value, optionally adding an offset, to determine a CFARthreshold value. Comparator KD compares each of the samples in the cellunder test to the threshold to determine whether a potential echo peakis present. As previously mentioned, a number of variations exist andwould also be suitable for use, albeit with different tradeoffs betweenperformance and computational complexity. One variation locates the ZUTat the right side of the shift register (for detecting rising edges) orat the left side of the shift register (to detect falling edges).

FIGS. 6A-6B are block diagrams of an illustrative edge-based echodetection process that may be implemented by the sensor controller. Amixer downconverts the receive signal to baseband or near-baseband. Afilter blocks the undesired frequencies from the downconversion process,and may further perform rectification and low-pass filtering to obtainthe envelope of the receive signal. An analog-to-digital converter ADCdigitizes the receive signal envelope, which is directed along twobranches.

Along one branch, a derivation element (“Norm. Deriv.”) determines thenormalized derivative as discussed previously. A comparator compares thenormalized derivative to a programmable threshold value, zeroing anysignal below the threshold. A peak detector identifies peaks in thenormalized derivative where it is above the threshold. In someimplementations, the peak detector generates a fixed-length pulse toindicate the position of each peak.

Along the other branch, a CFAR element derives a CFAR threshold from theRX signal, the threshold optionally including a programmable offset. Acomparator compares the RX signal to the CFAR threshold, producing anoutput signal that is asserted when the RX signal is above the thresholdand deasserted otherwise. An edge detector detects downward transitionsof the comparator output, upward transitions, or both, depending on theconfiguration of the sensor controller. In some implementations, theedge detector generates a fixed-length pulse to indicate the position ofeach edge.

A logical AND asserts an echo edge detection signal when both the RXsignal crosses the CFAR threshold and a peak is detected in thenormalized derivative. The AND element may be configured to generate afixed length pulse to indicate when the inputs have been simultaneouslyasserted.

In FIG. 6B, the echo edge detection signal is routed to both a timer anda peak amplitude element (“Max Track”). The timer is reset with eachtransmit burst, and with each assertion of the echo edge signal, itforwards a time of flight value to an (optional) correction element. Thepeak amplitude element identifies the peak amplitude of the RX envelopebetween assertions of the echo edge signal. Based on the peak amplitude,the correction element may adjust the time of flight values to reduce oreliminate any dependence of the time of flight on the echo amplitude.FIG. 7 shows an illustrative time-of-flight correction function that maybe implemented by a look-up table. The lookup table associates a deltato be added or subtracted from the time of flight value based on theamplitude of the echo, or more preferably, on the ratio between the peakof the normalized derivative and the peak amplitude of the echo.

Returning to FIG. 6B, the optionally-corrected time of flight may beprovided to a width determination unit which calculates echo pulsewidths. A framer (“Format/Buffer”) element collects the peak amplitude,optionally-corrected time of flight(s), and echo width values for eachdetected echo into a data frame for communication to the ECU.

Though the operations shown and described above are treated as beingsequential for explanatory purposes, in practice the process may becarried out by multiple integrated circuit components operatingconcurrently and perhaps even speculatively to enable out-of-orderoperations. The sequential discussion is not meant to be limiting.Further, the foregoing description has presumed the use of an I/O linebus, but other bus embodiments including LIN, CAN and DSI3 arecontemplated. These and numerous other modifications, equivalents, andalternatives, will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such modifications, equivalents,and alternatives where applicable.

What is claimed is:
 1. A controller for a piezoelectric transducer, thecontroller comprising: a transmitter to drive a piezoelectric element togenerate acoustic bursts; a receiver to sense a response of thepiezoelectric element to echoes of each acoustic burst; and a processingcircuit coupled to the transmitter and to the receiver, the processingcircuit operable to apply echo-detection processing to said response,said processing including: determining a derivative signal from theresponse; and detecting an echo based at least in part on a peak in thederivative signal indicating a falling edge in the response.
 2. Thecontroller of claim 1, wherein said detecting an echo is further basedon: comparing the response to an adaptive threshold to produce acomparator signal; and detecting an edge in the comparator signal. 3.The controller of claim 2, wherein said processing further includesderiving the adaptive threshold from the response signal using aconstant false alarm rate (CAFR) process.
 4. The controller of claim 1,wherein the derivative signal is determined by: forming a differencesignal between the response signal and a delayed response signal; takingan absolute value of the difference signal; and applying a local filterto the absolute value of the difference signal.
 5. The controller ofclaim 4, wherein the derivative signal is further determined by:applying a background filter to the absolute value of the differencesignal; and taking a ratio of an output from the local filter to anoutput of the background filter.
 6. The controller of claim 1, whereinsaid processing further includes: generating a pulse on an I/O line tosignal a time of flight of the echo.
 7. The controller of claim 1,wherein said processing further includes: generating a data frame thatincludes, for each echo detected in response to an acoustic burst, atime of flight for a falling edge of that echo.
 8. A controller for apiezoelectric transducer, the controller comprising: a transmitter todrive a piezoelectric element to generate acoustic bursts; a receiver tosense a response of the piezoelectric element to echoes of each acousticburst; and a processing circuit coupled to the transmitter and to thereceiver, the processing circuit operable to apply echo-detectionprocessing to said response, said processing including: determining aderivative signal from the response; and detecting echoes based at leastin part on peaks in the derivative signal indicating edges in theresponse.
 9. The controller of claim 8, wherein said detecting an echois further based on: comparing the response to an adaptive threshold toproduce a comparator signal; and detecting an edge in the comparatorsignal.
 10. The controller of claim 9, wherein said processing furtherincludes deriving the adaptive threshold from the response signal usinga constant false alarm rate (CAFR) process.
 11. The controller of claim8, wherein said processing further includes signaling an electroniccontrol unit to indicate one or more times of flight associated witheach echo, the one or more times of flight each corresponding to one ofsaid peaks in the derivative.
 12. The controller of claim 11, whereinsaid signaling indicates two times of flight associated with each echo,with a first time of flight corresponding to a rising edge peak and asecond time of flight corresponding to a falling edge peak.
 13. Thecontroller of claim 11, wherein said processing further includes:measuring peak amplitude of each echo; and adjusting the one or moretimes of flight associated with each echo based at least in part on thepeak amplitude of that echo.
 14. The controller of claim 13, whereinsaid adjusting is based on a ratio between the peak amplitude of thatecho and a peak in the derivative for an edge of that echo.
 15. Thecontroller of claim 8, wherein the derivative signal is determined by:forming a difference signal between the response signal and a delayedresponse signal; taking an absolute value of the difference signal; andapplying a local filter to the absolute value of the difference signal.16. The controller of claim 15, wherein the derivative signal is furtherdetermined by: applying a background filter to the absolute value of thedifference signal; and taking a ratio of an output from the local filterto an output of the background filter.
 17. A method of operating apiezoelectric-based sensor, the method comprising: driving apiezoelectric transducer to generate a burst of acoustic energy duringan actuation interval; during a measurement interval following theactuation interval, obtaining a response of the piezoelectrictransducer; and processing the response to sense echoes of the burst,said processing including: determining a derivative signal from theresponse; and detecting echoes based at least in part on peaks in thederivative signal indicating edges in the response.
 18. The method ofclaim 17, wherein said detecting an echo is further based on: comparingthe response to a constant false alarm rate (CFAR) threshold to producea comparator signal; and detecting an edge in the comparator signal. 19.The method of claim 17, further comprising: signaling an electroniccontrol unit to indicate a time of flight associated with each echo, thetime of flight corresponding to a falling edge peak in the derivative.20. The controller of claim 17, further comprising: signaling anelectronic control unit to indicate two times of flight associated witheach echo, with a first time of flight corresponding to a rising edgepeak and a second time of flight corresponding to a falling edge peak.